The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Ionized gas, or plasma, is commonly used during the processing and fabrication of semiconductor devices. For example, plasma can be used to etch or remove material from a substrate such as a semiconductor wafer, and to deposit material onto the substrate by PVD or CVD. Creating plasma for use in manufacturing or fabrication processes typically begins by introducing process gases into a processing chamber. The substrate is disposed in the processing chamber on a substrate support structure such as an electrostatic chuck or a pedestal.
The processing chamber may include a transformer coupled plasma (TCP) source coil, which receives a radio frequency (RF) power supplied by an RF power generator. A dielectric window, constructed of a material such as ceramic, is incorporated into an upper surface of the processing chamber. The dielectric window allows the RF power from the TCP source coil to be transmitted into the interior of the processing chamber. The RF power excites gas molecules within the processing chamber to generate plasma.
The plasma includes electrons and charged particles. The electrons, being lighter than the charged particles, tend to migrate more readily, causing a sheath to form at surfaces of the processing chamber. A self-biasing effect causes a net negative charge at inner surfaces of the processing chamber. This net negative charge is provided relative to ground (referred to as a direct current (DC) bias) and relative to a potential of the plasma (referred to as DC sheath potential). The DC bias is a difference in electrical potential between a surface within the processing chamber and ground. The DC sheath potential is a difference between the potential of the surface within the processing chamber and the potential of the plasma. The DC sheath potential causes the heavier positively charged particles to be attracted towards the inner surfaces of the processing chamber. Strength of this DC sheath potential at the substrate largely determines the energy with which the positively charged particles strike the substrate. This energy affects process characteristics such as an etch rate or a deposition rate.
A bias RF power source supplies a biasing RF power to the substrate support structure. The biasing RF power can be used to increase the DC bias and/or the sheath potential to increase the energy with which the charged particles strike the substrate. Variations in the biasing RF power produce corresponding variations in the DC bias and/or sheath potential at the substrate affecting the process characteristics.
A voltage control interface (VCI) including a pickup device and a signal processing circuit may be used to detect a RF peak voltage at the substrate support structure. The pickup device may be attached to the substrate support structure and receives the RF peak voltage (i.e., RF bias voltage). The signal processing circuit is connected to the pickup device and converts the RF peak voltage into an analog signal that has a magnitude proportional to the peak value of the RF voltage under detection. When the bias RF system is operating on voltage mode, the biasing RF power is adjusted based on the detected RF peak voltage so that the bias RF voltage is regulated to its setpoint given in the process recipe.
A voltage sensor or pick-up device of a VCI may include a capacitive voltage divider for RF voltage detection on a corresponding channel. The VCI may include circuitry for signal conditioning and processing of a voltage signal received on the channel. The voltage sensor has a dynamic range that is typically limited to less than 40 db with reduced accuracy at low voltages. For example, the voltage sensor may have a dynamic range of 33.6 db from a 25 volt (V) peak to a 1200V peak with accuracy of t (1V+1.5% of a National Institute of Standards and Technology (NIST) reference value).